1. Field of the Invention
The present invention relates to the field of Magnetic Resonance Imaging and more specifically to the field of selective imaging of spectroscopic components with Magnetic Resonance.
2. Description of Related Art
The nucleus of an atom in a molecule of a material placed in a magnetic field will resonate when irritated with radiofrequency (RF) radiation of the proper frequency.
The nucleus will also produce a magnetic resonance (MR) response signal at the same resonant frequency. This resonant frequency, known as the `Larmor` frequency, is a function of the applied magnetic field, the composition of the nucleus and other parameters. Different nuclei have different resonant frequencies, For example, in an applied magnetic field of 1.5 Tesla, silicon resonates at 12.7 MHz and hydrogen being at 63 MHz. Different nuclei, such as hydrogen and silicon may be differentiated by employing different RF radiation frequencies since they are far apart. This cannot be the case, however, with chemical species which are close on the frequency band.
The effects of the electron cloud of the molecule which the nucleus is a part, `electron shielding`, and the effects of neighboring molecules also affect the resonant frequency. These effects cause the resonant frequency to be slightly shifted from what it would be if they were not present, known as `chemical shift`. These shifts may be on the order of a few Hertz to 100 kHz. Each chemical species creates a MR spectrum having peaks of increased amplitude at specific frequencies, characteristic of the species. Chemical species may be classified by a few of their substantial peaks. For example, in a 1.5 Tesla magnetic field, Hydrogen atoms in water have a resonant frequency at 63.8 MHz, whereas the Hydrogen atoms in lipids have a resonant frequency approximately 100 Hz higher.
A problem arises in imaging different chemical species of the same nuclei simultaneously. Since most MR imaging methods employ the use of a spatially changing magnetic field gradient to encode the location of a resonating nuclei, `nuclear spins` or simply `spins`, to as a frequency change in an MR response signal which is detected. Fourier transforms then map each frequency to a location. This causes the slight differences in resonant frequency of the different chemical species to be lost. An MR response signal from a first chemical species will have the same frequency and mimic a MR response signal from a second chemical species at a different location. This also leads to artifacts if the resonant frequencies of both species within the frequency range caused by the magnetic field gradient.
Most of the protons which generate an MR signal in the body arise from water or lipids. Because water and lipids have different chemical shifts, MR signals from water and lipids, occur at slightly different Larmor frequencies and theoretically can be separated.
Several methods have been previously disclosed for the simultaneous detection of two or more chemical species in a magnetic resonance image. One method, known as chemical shift imaging, requires the addition of a chemical shift dimension to a conventional two-dimensional magnetic resonance imaging procedure. This is accomplished by acquiring MR response signals at a series of uniformly incremented echo times. The frequency of each magnetic MR response signal is defined by the chemical shift of each chemical species. Since each chemical species has a unique MR response signal frequency, the phase evolution of transverse so spin magnetization the excitation, proceeds at a unique rate. The rate of phase evolution (i.e. the frequency) of each chemical species is determined by Fourier transforming the MR response signals.
W. Thomas Dixon has described in "Simple Proton Spectroscopic Imaging", Radiology, Vol. 153, p. 189-194 1984, a second method in which images are made of a selected spectral component. In this method, two images are acquired under identical conditions, except for the echo time which is chosen so that transverse spin magnetization from a selected chemical species has an additional phase shift. The complex difference between the acquired MR response signals is then, calculated. Resonances which have not acquired a substantial phase shift responsive to the additional echo time will be substantially cancelled upon subtraction. MR response signals which have acquired a .phi. phase shift, or greater, however, will add constructively upon subtraction.
Currently, imaging of different chemical species, such as silicone gel, is complicated by the fact that the signals acquired in conventional images have chemical shift artifacts and it is difficult to discriminate small amounts of gel from edema and healthy tissue. There is a need for a method of simultaneously imaging several chemical species dearly with MR imaging.